Monolithic integration of opto-electronic devices requires the development of techniques to produce extremely smooth etched laser facets. Such facets are essential to the development of many photonic devices, as well as being used in turning-mirror fabrication, and etched facet lenses. Etched facets have also been used to produce monolithic unstable resonator designs with low divergence and single longitudinal mode operation from a broad area semiconductor laser. Additionally, etched facets are being considered for wafer-level facet coating and wafer level testing to reduce required part handling. While the wide-stripe resonators fabricated to date have proven capable of matching the performance of conventional cleaved facet lasers, a further reduction of facet roughness is needed for advanced photonic devices. Low surface roughness is essential to maximizing facet reflectivity, minimizing beam divergence and scattering loss, and in turning mirrors, minimizing backscatter. Extremely low surface roughness is also essential to the fabrication of quantum wire lasers.
The ideal technique for evaluating etched facets would have the following characteristics: (a) uses only commonly available laboratory equipment; (b) evaluates facets independently of other processes (i.e., test results of different facet etching techniques are not biased by other variables such as starting wafer quality or the contact resistance of the diode); (c) is sensitive to relatively small changes in facet quality from run to run; (d) is sensitive to all sources of facet roughness, including poor photolithography or linewidth control, or grainy photoresist or masking materials; (e) is quick and requires little sample preparation; and (f) is nondestructive, and can be used as a quality check on the facets before further processing is performed on a wafer with unknown facet quality. On these counts, the technique described herein; (a) requires only a scanning electron microscope (SEM) and a low-cost photo scanner; (b) acquires the actual facet profile and does not infer roughness from ambiguous electro-optic measurements; (c) can reproducibly detect changes of less than 10% from run to run, unlike subjective facet evaluation which cannot accurately detect changes in roughness less than 30%, (d) detects even slow deviations from the intended facet profile (due to lithographic resolution limitations) which go unnoticed when examining typical SEM photographs; (e) requires no sample preparation and takes less than two hours to perform all aspects of the analysis shown below: and (f) is completely nondestructive (although resolution is improved by applying a conductive coating where nondestructive analysis is not required). Previously, facet quality has either been inferred by comparing the slope efficiency of an etched-facet laser to a similar cleaved-facet laser, or evaluated qualitatively by estimating the size of sidewall striations. The technique takes advantage of the "tilt adjustment" feature on the SEM to provide y scale expansion, simultaneously capturing information from a wide area in the x axis, while giving high magnification in the y axis. The resulting SEM photo is then digitized to provide the actual facet profile. This technique provides what we believe to be the first quantitative evaluations of etched facet roughness, which should prove helpful in comparing devices fabricated by different processes, as well as predicting device performance using techniques of numerical analysis. This technique takes the Fourier transform of the surface profile, and shows that the surface roughness is composed of three distinct components, resulting from different physical effects. We believe this method has broad applicability to process control and accurate numerical analysis.